Allow custom markers with values

Allow custom markers to have a value like kubebuilder markers.

```go
type Config struct {
// +kubebuilder:default=25
MaxOpenConns int `json:"maxOpenConns,omitempty"`
}
```

Motivation is to define an `env` custom marker to generate an extra column with the equivalent environment variable name.

```go
type Config struct {
// +env="MAX_OPEN_CONNS"
MaxOpenConns int `json:"maxOpenConns,omitempty"`
}
```

I tried implementing this using the existing `processor.customMarkers` config and modified templates like the examples, but it seems like the markers are not matched if they are followed by an equals sign.